1. Field of the Invention
The present invention relates to a method and apparatus for developing electrostatic latent images formed in electrophotographic process, electrostatic recording process and other similar process.
2. Description of the Prior Art
In the technical field of image-forming for electrophotography and electrostatic recording, there are known and used various methods to visualize, that is, develop the latent image electrically formed on a latent image carrier such as a so-called photosensitive medium made of photoconductive material, an electrostatic recording material and the like. According to these conventional methods, the latent images are visualized, i.e. developed by applying onto the latent image carrying surface electroscopic particles, that is, developing particles which are more or less selectively attracted or repulsed by the electrostatic charge of the latent image, although there is some difference in actual procedure depending upon whether a direct reproduction or a reversal reproduction is desired. The above-mentioned type of developing or electroscopic particle is generally called toner and well-known in the art. For a direct reproduction, the developing particles adhere to the area of the latent image. For a reversal reproduction, the developing particles adhere to the area out of the latent image.
As such developing method, hitherto there are widely known two types of developing methods. One is a so-called dry developing method wherein a developer in the form of dry powder is used. The other is a wet developing method wherein a dispersion of developing particles in liquid is used.
For example, magneto-brush method as disclosed in U.S. Pat. No. 2,874,063, cascade method as disclosed in U.S. Pat. No. 2,618,552 and powder clouding method as disclosed in U.S. Pat. No. 2,221,776 are known as representative of the dry developing method.
One typical wet developing method hitherto known is a method wherein the electrostatic latent image carrying surface is brought into contact with a so-called liquid developer containing developing particles dispersed in a dielectric liquid carrier having a volume resistance more than 10.sup.10 .OMEGA.cm and a permittivity less than 3 (for example, paraffin hydrocarbons). When contacted, the developing particles, i.e. toner particles are adsorbed by an attraction force onto the electrostatic latent image on the image carrying surface and thereby development of the latent image is effected.
All of the known methods for developing electrostatic latent images are in common with each other in the fact that there are used such developing particles that are more or less selectively attracted or repulsed by the electrostatic charge of the latent image and their deposition effect is made use of to visualize the latent image.
However, these conventional developing methods which are based upon the deposition of the developing particles, have various problems and drawbacks in view of their practical use. For example, in order to efficiently effect depositing such developing particles, it is required to charge the particles with a sufficient electrostatic charge enough to allow the particles to deposit in a short time. Furthermore, in order to produce images uniform in quality, the individual particles have to be charged uniformly. These requirements of developing particles can be satisfied only by a high standard preparing technique and a high cost.
Another problem is found in that a suitable control of the density of developer and a uniform supply of the developer onto the latent image carrying surface are required to attain a uniform deposition with a stable density of the developing particles on the electrostatic latent image. To solve the problem it has been proposed in the art to supply the developer in a form of jet flow. But, this method has a particular drawback that it is very difficult to control the flow of the jet uniformly.
The above described problems involved in the conventional developing methods will be intensified when developments should be carried out with a higher efficiency and in a shorter time. The deposition of developing particles in the wet developing process, i.e. liquid developing process is generally considered to be based on the phenomenon of electrophoresis of the electrically charged developing particles. In general, the migration speed of charged particles is intrinsically very slow. Therefore, for a speed-up of the development, it is absolutely necessary to increase the electric charge on the developing particles, to supply a greater amount of developing particles with a higher efficiency and also to intensify the electric field up to a sufficiently higher level.
However, preparation of such developing particles having a higher and uniform electric charge is extremely difficult to do. Moreover, in order to enable to supply the developer with a higher efficiency, a developer containing developing particles in higher concentration must be used while supplying it in a form of high speed jet flow or employing a high speed coating roller. But, when the concentration of developing particles in a developer is increased, there often occurs another problem called "fogging" phenomenon which makes the non-image portion dirty due to the undesirable adhesion of developing particles other than those deposited in the image portion by the electrostatic attraction. Therefore, the use of high concentration of developing particles is limited. To eliminate the problem of this "fogging" it is known and used to apply a bias, the polarity of which is opposite to that of the particles, to the conductive rollers or electrodes used in the developing station. But, since the electric field used for the deposition of developing particles is usually opposite to that used for the elimination of fogging, the use of this solution is restricted within limited cases as a matter of course.
It is true that an increase in efficiency of development may be attained by increasing the intensity of the electric field and by increasing accordingly the migration speed of the developing particles. However, it has also some severe limitations to make the electrode distance between the developing electrodes small as required for this purpose as well as to increase the potential of the latent images as required.
On the other hand, when the liquid developer is supplied with a high speed, the ununiformity of flow of the liquid developer will be enhanced thereby because of a rapid movement of the liquid containing developing particles dispersed therein. As a result, the developed images lack uniformity and sharpness. To produce developed images of good quality sufficient for practical use, it is absolutely necessary to precisely accomplish the uniformity of flow of the developer to be supplied.
In summary, all of the developing methods hitherto known necessitate a high standard of technique regarding the control of characteristics of developer and of its supply. Also, it is one the important drawbacks of the known developing methods that there is a particular difficulty to eliminate the so-called marginal effect. Marginal effect is known as such a phenomenon that the portion of a developed image which should have the same density does not have the same density but becomes thicker at its marginal portions and thinner at its center. Various methods are known and used to reduce the marginal effect. For example, it has been proposed to form the electrostatic latent image in a form of mesh points. Also, it is known to dispose a flat plate electrode in parallel with and very close to the surface of electrostatic latent image so that the development of the latent image may be carried out decreasing the intensity of the electric field applied to the marginal portion of the electrostatic latent image. However, all the known methods were found to be unsatisfactory to eliminate the marginal effect completely.
For the sake of reference, there will now be mentioned the prior art apparently similar to the invention.
U.S. Pat. No. 2,297,691 discloses a developing method in which electroscopic particles (that is a so-called toner) are supplied to an electrostatic latent image carrying surface and any excess toner remaining on the produced image is removed out of the image carrying surface by blowing air (see lines 27-44 in the right column on page 6, lines 12-17 at the right column on page 3 and FIGS. 3 and 4).
U.S. Pat. No. 3,276,896 discloses another developing method in which after developing a latent image with a liquid developer, a liquid medium suspension containing substantially none of electroscopic developing particles is applied to the developed image so as to solve the problem of adhesion of developing particles on the non-image portion which makes the image dirty (see lines 19-27 in column 5, lines 20-23 in column 2 and FIGS. 1 and 2).
The latter mentioned invention, namely the invention disclosed in U.S. Pat. No. 3,276,896 may be considered at a first glance to be similar to the present invention. However, there is a distinct difference therebetween. The primary object of the prior invention is to eliminate the problem of developed images being stained with excess developing particles, that is, the problem of fogging. To this end, the developing method according to the prior invention comprises two steps, namely a first step of developing an electrostatic latent image with electrically charged developing particles based upon the phenomenon of electrophoresis of the particles and a second step of cleaning off such developing particles unnecessarily adhered to the non-image portion. In contrast with the prior invention, the object of the present invention is to provide a developing method which enables the carrying out of a liquid development, in particular, at a higher speed. According to the present invention, the supply of developing particles to an electrostatic latent image carrying surface at its first step of the method is carried out not to have the particles deposit based on the electrophoresis of the particles relative to the electrostatic latent image but to have the particles adhere to the image carrying surface mainly depending upon the cohesive force between the particles and the inter-molecular force between the particle and the image carrying surface, independently of the electrostatic latent image. The visualization, i.e. development of the electrostatic latent image takes place at the second step of the method only when a liquid is supplied to the developing particles on the image carrying surface. According to the method of the present invention, the problem of the known developing methods caused by the fact that the known methods are based on the electrophoresis action of the electrically charged particles, can be substantially eliminated.
As will be understood from the foregoing, there is a fundamental difference in object as well as in effect of the invention between the above mentioned prior inventions and the present invention. Therefore, the present invention is entirely novel with respect to the prior art.
An apparent similarity of the developing methods and apparatus of the prior art to the present invention is attributable to the fact that the former also has a first step of supplying electroscopic particles and a second step of removing excess developing particles. However, in all the known methods as particularly described above, the visualization, i.e. development of the latent image primarily takes place at the first step, namely at the time when the toner is supplied. On the contrary, in the method of the present invention, the development of the latent image mainly takes place at the second step where a liquid is supplied. In view of these points, the present invention differs from the prior art in object and in effect.